55f> 



The Ascent of Water in Trees. 



some kind or other must be exercised in the wood, for which the 

 presence of active living cells is essential. In support of this it has 

 been shown that the production of wood in a slowly growing tree is 

 greater than is necessitated by mechanical requirements. In other 

 words, the production of new wood is largely determined by the length 

 of time during which the wood-parenchyma can remain active. 



There is no known means by which these cells can directly pump 

 water in a definite direction, although the existence of a power of 

 absorbing and exuding water under pressure has been empirically 

 determined to exist in the living wood of cut branches. It is suggested 

 that the wood-parenchyma cells by the excretion and re-absorption of 

 dissolved materials may bring into play surface-tension forces within 

 the vessels of sufficient aggregate intensity to maintain a steady 

 upward flow, and to keep the water of the Jamin's chains in the 

 vessels in a mobile condition ready to flow to wherever suction is 

 exercised upon it.* The rapid rates of diffusion required for such 

 action do actually exist in the wood-parenchyma cells. 



It appears that the terminal branches of trees at heights of from 

 22 to 44 feet above ground exhibit little or no power of bleeding in 

 spring. Possibly in such trees the pumping action is only used or 

 developed in the wood of the older stems, or is only exercised when trans- 

 piration is active, and when the water-columns in the vessels attain a 

 definite size relatively to the wood-parenchyma cells. The importance of 

 the Jamin's chain in the vessels is that it renders a staircase pumping 

 action possible, and enables the water to be maintained in the vessels 

 in a labile condition, ready to flow to any point where moderate suction 

 is exercised. This pumping action being diffused and probably 

 regulated, need not produce any high pressure of exudation at the 

 terminal branches of tall trees, which, in fact, appears always to be 

 absent at high levels. 



* Surface-tension actions would be possible in the absence of air-bubbles 

 \vherever the wood-parenchyma cells contained oil or any other substance non- 

 miscible with water, as they often do. 



